Post tensioned Transfer Girder 1

[slickdeals]

Folks,
I am looking for some help and guidance in the design of a 58 foot long PT transfer girder carrying 4 floors. Are there any rules of thumb for sizing, examples for design that might be useful.

I don't want to plug numbers into a program without understanding the design procedures and pitfalls.

Thanks

 

[271828]

No sizing rules of thumb that I know of anyway. The trickiest part of this will be selecting PT forces that are staged as more floors are piled on. Check shear early on. There's also the issue of creep deflection. If your column sinks over time, that could cause serious problems.

 

[fryed911]

If it's a deep beam, you really need to use strut-and-tie methods to adequately calc the forces.

 

[csd72]

Deflection of the transfer beam will effect the moments in the frame over. You need to take this into account.

 

[onetimedeal]

Try a 36"x48" beam with a PT force of 2,500 kips. If you ae using ADAPT PT, the program should adjust the PT force for you. If you are using RAM or other programs, you may want to do a trial-and-error in order to get an ultimate design.

Like others said, you have to do "stage stressing" for this transfer beam. The reason is not to apply too much post-tensioning force when you do not have that much dead load to balance. You best dead load balance is around 60% to 80% of dead load. If you design unbonded tendons, you can specify percentage of tendons to be stress at each stage. If you decided to use bonded tendons, it is better to stress one tendon to 100% at a time rather than stress 50 % of total force. Typically, you can specify two stage stressing for a 4-floor girder.

In addition, you may want to use bonded tendons for this transfer girders because ACI 318 allows you to design PT force using much higher steel stresses. If you decided to use bonded tendons, you may want to check PT supplier's catalog for sizes of the anchorages and make sure they will fit in your beam's end section. Your end drap may be affected accordingly, too.

 

[ron9876]

58 feet is long. How much load? Where is load on the span? As mentioned above the staging is critical. Need to do a full design of the beam for all stages and check mild and pt reinf. Have to use trial and error to come up with a design that fits within the architecture.

The best advice that I have received in my career was when I was a couple of years out of school and the owner of the company that I worked for told me that you need to be aware when something looks funny or overly complex because you will normally have something laid out wrong. It has always amazed me have effective this simple advice is. I recommend that you look for a different solution that doesn't require a 58' transfer beam.

 

[AUCE98]

For this type of transfer girder, I would also think that a bonded system is much more advantageous. Couple of reasons why, one as indicated above, you are permitted to use a higher stress at ultimate load if it is bonded. Secondly, you may minimize the amount of bonded mild steel required in the beam by utilizing the bonded pt. Thirdly, there is only localized loss if a tendon is drilled into, or cut. Additionally, it provides much better long term corrosion protection. Your drapes will be significantly less due to the size of the pt ducts, so do check with a local PT supplier regarding the detailing. Also, bonded tendons utilize 0.6" strand rather than 0.5" strand that is used for unbonded monostrand. This will provide approximately 30% increase in force per strand. Take a look at

http://www.vsl.net,

they have a bonded system.

Also, deflections are typically limited to L/1660